Interaction of plasma functionalized TiO2 nanoparticles and ZDDP on friction and wear under boundary lubrication

Tribochemical interaction of zinc dialkyl dithiophosphate (ZDDP) with titanium dioxide nanoparticles and plasma functionalized TiO2, to ascertain their synergist effects on tribological performance of steel test pairs under boundary lubricated sliding conditions in a mineral base oil, was investigated. A pin on reciprocating flat test configuration was utilized to determine the tribological properties of the individual additives and their mixtures in the oil. Specifically, friction coefficients, wear losses, and electrical contact resistance (ECR) data from seven different formulations were determined. Tribofilms, generated during sliding, were subjected to extensive surface analysis using X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) to determine the nature and the extent of tribochemical interactions. Results from these tribological tests, coupled with chemical characterization of the tribofilms, revealed that both ZDDP and TiO2 nanoparticles (by themselves) form very protective tribofilms on the rubbing surfaces. However, when used as a mix in the mineral oil, they surprisingly behave antagonistically hence resulting in severely increased wear. In contrast, TiO2 nanoparticles coated with boron rich plasma films showed relatively better anti-wear performance than when the uncoated ones were used singularly in the oil or as a mix with ZDDP.